问题
Hello and thanks in advance for your time.
In my code I am making various requests to AWSSQS which all return AWSTask. I have found working with these AWSTask objects to be very difficult while also trying to keep all the logic specific to AWS in a single class so I can easily switch to a different cloud service if need be.
Ideally, what I would like to do is execute a series of AWS tasks asynchronously in a serial fashion. Normally I would just add tasks to a custom Serial Dispatch Queue but since The AWSTask objects are themselves asynchronous tasks, I can't do that.
Here is a simple example that illustrates the problem I am having. It doesn't have any real world purpose but it does a good job illustrating the problem. Below, I have code to create a SQS queue, send a message to a SQS queue, receive a message from an SQS queue, and delete a SQS queue. Let's say I want to do those four things in a serial, asynchronous fashion. In other words, I want to make sure the previous task succeeded before attempting the next task.
ViewController
DispatchQueue.global(qos: DispatchQoS.QoSClass.userInitiated).async {
awsClass.runTest()
DispatchQueue.main.async {
print("Test Finished")
}
}
AwsClass
public func createQueue(){
guard let createQueueRequest = AWSSQSCreateQueueRequest() else{fatalError()}
createQueueRequest.queueName = "TestQueue"
sqs.createQueue(createQueueRequest).continueWith(block: {(task) -> AnyObject? in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
self.queueUrl = task.result!.queueUrl!
print("created queue at: \(self.queueUrl!)")
}
return nil
})
}
public func deleteQueue(){
if queueUrl != nil {
guard let deleteQueueRequest = AWSSQSDeleteQueueRequest() else{fatalError()}
deleteQueueRequest.queueUrl = queueUrl
sqs.deleteQueue(deleteQueueRequest).continueWith(block: {(task) -> AnyObject? in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
print("queue sucessfully deleted from \(self.queueUrl!)")
self.queueUrl = nil
}
return nil
})
}
else{
print("Queue has already been deleted")
}
}
public func sendMessage(messageData: String, toConnectId: String) {
guard let sendMessageRequest = AWSSQSSendMessageRequest() else{fatalError()}
sendMessageRequest.queueUrl = toConnectId
sendMessageRequest.delaySeconds = 0
sendMessageRequest.messageBody = messageData
sqs.sendMessage(sendMessageRequest).continueWith(block: {(task) -> AnyObject? in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
print("successfully sent message to \(toConnectId)")
}
return nil
})
}
public func receiveMessage(){
guard let receiveMessageRequest = AWSSQSReceiveMessageRequest() else{fatalError()}
receiveMessageRequest.queueUrl = self.queueUrl
receiveMessageRequest.maxNumberOfMessages = 1
sqs.receiveMessage(receiveMessageRequest).continueWith(block: {(task) -> AnyObject? in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
let message = (task.result?.messages?.first)!
print("successfully received message with body: \(message.body ?? "failed")")
}
return nil
})
}
public func runTest(){
let mySerialQueue = DispatchQueue(label: "mySerialQueue")
mySerialQueue.sync {
self.createQueue()
}
mySerialQueue.sync {
self.sendMessage(messageData: "test", toConnectId: "https://someUrl")
}
mySerialQueue.sync {
self.receiveMessage()
}
mySerialQueue.sync {
self.deleteQueue()
}
}
Since the AWSTasks are asynchronous with completion functions, the code quickly makes all four calls and then the completion functions are called whenever those tasks finish. Instead I want the completion function of the first task to finish before the next task begins.
回答1:
The AWSTask objects are meant to be "chained" together. Documentation can be found here: http://docs.aws.amazon.com/mobile/sdkforios/developerguide/awstask.html
A small example here:
sqs.createQueue(/* parameters */).continueWithSuccess(block: {(task) -> Void in
// Success
return sqs.sendMessage(/* parameters */)
}).continueWithSuccess(block: {(task) -> Void in
// Success
return sqs.receiveMessage(/* parameters */)
}).continueWithSuccess(block: {(task) -> Void in
// Success
return sqs.deleteQueue(/* parameters */)
})
回答2:
Alright, so I found a solution to my question. It works exactly as desired but it does so in this nasty chain of completion functions. If anyone knows a more elegant solution, I am all ears!
ViewController
print("Starting Test")
DispatchQueue.global(qos: DispatchQoS.QoSClass.userInitiated).async {
atomConnector.runTest(completion: {
print("test finshed")
})
}
AwsClass
public func createQueue(completion: @escaping () -> Void){
guard let createQueueRequest = AWSSQSCreateQueueRequest() else{fatalError()}
createQueueRequest.queueName = "TestQueue"
sqs.createQueue(createQueueRequest).continueWith(block: {(task) -> Void in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
self.queueUrl = task.result!.queueUrl!
print("created queue at: \(self.queueUrl!)")
completion()
}
})
}
public func deleteQueue(completion: @escaping () -> Void){
if queueUrl != nil {
guard let deleteQueueRequest = AWSSQSDeleteQueueRequest() else{fatalError()}
deleteQueueRequest.queueUrl = queueUrl
sqs.deleteQueue(deleteQueueRequest).continueWith(block: {(task) -> Void in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
print("queue sucessfully deleted from \(self.queueUrl!)")
self.queueUrl = nil
completion()
}
})
}
else{
print("Queue has already been deleted")
}
}
public func sendMessage(messageData: String, toConnectId: String, completion: @escaping () -> Void) {
guard let sendMessageRequest = AWSSQSSendMessageRequest() else{fatalError()}
sendMessageRequest.queueUrl = toConnectId
sendMessageRequest.delaySeconds = 0
sendMessageRequest.messageBody = messageData
sqs.sendMessage(sendMessageRequest).continueWith(block: {(task) -> Void in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
print("successfully sent message to \(toConnectId)")
completion()
}
})
}
public func receiveMessage(completion: @escaping () -> Void){
guard let receiveMessageRequest = AWSSQSReceiveMessageRequest() else{fatalError()}
receiveMessageRequest.queueUrl = self.queueUrl
receiveMessageRequest.maxNumberOfMessages = 1
sqs.receiveMessage(receiveMessageRequest).continueWith(block: {(task) -> Void in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
let message = (task.result?.messages?.first)!
print("successfully received message with body: \(message.body ?? "failed")")
self.deleteMessage(receiptHandle: message.receiptHandle, completion: completion)
}
})
}
public func deleteMessage(receiptHandle: String?, completion: @escaping () -> Void){
guard let deleteMessageRequest = AWSSQSDeleteMessageRequest() else{fatalError()}
deleteMessageRequest.queueUrl = self.queueUrl
deleteMessageRequest.receiptHandle = receiptHandle
sqs.deleteMessage(deleteMessageRequest).continueWith(block: {(task) -> Void in
if task.error != nil {
print(task.error!)
}
else if task.result != nil {
print("successfully deleted message with receiptHandle: \(receiptHandle)")
completion()
}
})
}
public func runTest(completion: @escaping () -> Void){
self.createQueue(completion: {
self.sendMessage(messageData: "test", toConnectId: "https://someUrl", completion: {
self.receiveMessage(completion: {
self.deleteQueue(completion: {
completion()
})
})
})
})
}
来源：https://stackoverflow.com/questions/44688027/what-is-the-proper-way-to-work-with-awstask-objects-in-swift